Flat-panel displays are widely used in computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed in an array over a display substrate to display images, graphics, or text. For example, liquid-crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals. Organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the electrical current. Each pixel usually includes three or more sub-pixels emitting light of different colors, for example red, green, and blue.
Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Typically, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix OLED display, each control element includes two transistors (a select transistor and a drive transistor) and one capacitor for storing a charge specifying the desired luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single-transistor circuit. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) in a semiconductor layer formed on a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors, Additional layers, for example insulating dielectric layers and conductive metal layers are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, structures, or wires.
In any display device it is important that light is uniformly displayed from the pixels arranged over the extent of the display when correspondingly controlled by a display controller to avoid visible non-uniformities or irregularities in the display. As display size and resolution increase, it becomes more difficult to manufacture displays without any pixel defects and therefore manufacturing yields decrease and costs increase. To increase yields, fault-tolerant designs are sometimes incorporated into the displays, particularly in the circuitry used to control the pixels in the display or by providing additional redundant pixels or sub-pixels.
Numerous schemes have been suggested to provide pixel fault tolerance in displays. For example, U.S. Pat. No. 5,621,555 describes an LCD with redundant pixel electrodes and thin-film transistors and U.S. Pat. No. 6,577,367 discloses a display with extra rows or columns of pixels that are used in place of defective or missing pixels in a row or column. U.S. Pat. No. 8,766,970 teaches a display pixel circuit with control signals to determine and select one of two emitters at each sub-pixel site on the display substrate.
Furthermore, in flat-panel displays using thin-film transistors formed in an amorphous or polysilicon layer on a substrate, the additional circuitry required to support complex control schemes can further reduce the aperture ratio or be difficult or impossible to implement for a particular display design.
There remains a need, therefore, for a design and manufacturing method that enables fault tolerance in a display without compromising the aperture ratio of the display or limiting display design options.